1. Field of the Invention
This invention relates generally to circuit breaker units and more particularly to circuit breaker units for large capacity circuit breakers, such as draw-out type circuit breakers.
2. Description of the Related Art
Circuit breaker units comprise, in part, circuit breaker backmolds which are commonly used as mounting surfaces for turnable joint mounts, current transformers and metering transformers.
The turnable joint mounts connect with the field serviceable connections to which the circuit breaker protects against short circuit and overload situations. The backmold is the main interface between the circuit breaker and the field serviceable connections. Therefore, the circuit breaker backmold must be reliable, safe and cost effective.
On large capacity circuit breakers, such as a draw-out type circuit breaker, extreme forces are placed along the horizontal axis of the backmold when engaging and disengaging (also known as "racking") the circuit breaker and when connections are made between the turnable joint mounts and field service connections, which can weigh in excess of 200 lbs. Force is also exerted on the backmold when the circuit breaker, often rated up to 6,000 amps, is tripped. Alone or in combination, these forces can cause deflection or bowing in a conventional backmold, which is made from thermoset materials, such as Bakelite.RTM., a phenolic resin, that are heated in compression or injection molds.
Electrical clearance parameters and the dimensions of the circuit breaker enclosure limit the ability to add a bracing element to the backmold in order to provide greater strength and resist deflection and bowing. Therefore, in order to produce a backmold with sufficient strength, the compression or injection molds used to produce the conventional backmold must be of a suitable depth. Typical Bakelite.RTM. backmolds have a thickness of 4-5 inches and therefore, it is not feasible to add a brace to the backmold because of the limited space within the circuit breaker unit.
Other disadvantages with the use of conventional backmolds include the difficulty experienced when making modifications to the backmold. Conventional backmolds produced by compression or injection molding processes require changes in the mold when modifications are to be made to the backmold. Changes in the design of the mold are extremely costly because of the complexity of the mold. In contrast, less complex molds are used in manufacturing pultrudled backmolds and a majority of the features on the pultruded backmold can be post-machined. Therefore, when changes need to be made to a pultruded backmold, changes are made in the machining process and not necessarily the mold itself.